1. Technical Field
This invention relates to a line smoothing circuit capable of indicating a straight line smoothly on a raster scan type graphic display unit.
2. Prior Art
A raster scan type graphic display unit is an apparatus for displaying a graphic form while controlling the luminance of a CRT raster on the basis of the graphic data which are expressed by an aggregate of line vectors defined by the coordinates of a starting point (x.sub.i, y.sub.i) and a terminal point (x.sub.i+1, y.sub.i+1) as shown in FIG. 8.
The controlling of the luminance is done by carrying out the computation, ##EQU1## on the basis of a value of the coordinates of a starting point (x.sub.i, y.sub.i) and a terminal point (x.sub.i+1, y.sub.i+1), which are expressed by the graphical data, by a computing means A, outputting a "1" signal as the image data into the addresses (X, Y) in which the integer parts exist to store the "1" signal in an image memory B, and reading the signal synchronously with a read signal from a read control means C, as shown in FIG. 7.
However, since one straight line is expressed by discrete numerical values, the straight lines (FIGS. 9A and 10A) extending diagonally near the horizontal and vertical axes are displayed in a stepped manner as shown in FIGS. 9B and 10B. Accordingly, bent parts occur, and although these bent parts are difficult to be seen because they are so small, the lines nevertheless do not appear as smooth straight lines.
Such a problem can be solved by increasing the density of the display dots but an increase in the density of the display dots makes it necessary to increase the capacity of an image memory to a great extent.